The present invention is directed to receiver systems for detecting spread-spectrum signals in the outputs of a multi-element antenna system. It is particularly applicable to systems, such as direction-finder and other high-gain arrangements, in which the phase relationships between the outputs of the individual elements are critical.
Spread-spectrum signals are used for secure communications. In this type of signal, the information is spread throughout a broad frequency band and is typically buried in ambient noise and signals from other sources. The frequency spreading is accomplished by phase modulating a carrier in discrete shifts, typically of 180.degree. or of various multiples of 90.degree., in a pseudo-random fashion. This pseudo-random modulation eliminates the spectral line at the carrier frequency, and it occurs at a rate that is high enough to spread the frequency content of the original signal throughout a wide range so that the bandwidth of the resultant spread-spectrum signal is as much as twenty percent or more of the carrier frequency. If the pseudo-random series of phase changes is known at a receiving station, it is possible to "squeeze" the spread-spectrum signal back into a narrow-band signal by reversing the phase shifts that originally caused the signal spectrum to be spread. This raises the signal out of the noise and makes it possible to detect it.
A method that we invented for detecting the presence of a spread-spectrum signal without knowing the pseudo-random shift pattern in advance is described in our U.S. Pat. No. 4,247,939 for a Spread Spectrum Detector. That patent describes the mixing of the received signal with the output of a swept-frequency, or chirped, local oscillator, squaring the result, and applying the squared signal to a dispersive delay line that has a linear relationship of delay to frequency. The relationship is such that the ratio of frequency difference to the associated difference in delay is twice the time rate of frequency change of the chirped oscillator. It is twice the chirped-oscillator rate because the squaring step doubles the frequency of the mixed signal.
This system acts as a special type of compressive receiver, first compressing the spread-spectrum signal in frequency by squaring it and then compressing the frequency-compressed signal in time by operation of the dispersive delay line. The result of the frequency and time compressions is to produce an output signal from the delay line in which the result of the spread-spectrum signal is a short-duration pulse that occurs at a time during the chirp-oscillator sweep that is indicative of the center frequency of the spread-spectrum signal. Concurrently, the received signal, without mixing with the chirp signal, may be squared and applied to a narrow-band filter that is set to the frequency indicated by the time at which the output pulse occurs. The output of this filter may then be demodulated or subjected to other processing.
It will be appreciated that the presence of strong narrow-band signals in the input of the squaring circuit can seriously degrade its performance. Accordingly, our above-mentioned patent discloses the use of a device for reducing the contributions of narrow-band signals in the input of the squaring circuit. In one version, a contiguous comb filter receives the input signal and divides it into contiguous, narrow frequency bins, frequency components falling within the respective bins appearing at separate comb-filter output ports. Any spread-spectrum signal is distributed among the many comb-filter output ports and contributes only minimally to any individual output. A strong narrow-band signal, however, will appear principally in only one or two of the outputs. Accordingly, individual amplitude limiters at the output ports of the comb filter can restrict the signal contributions of strong narrow-band signals without appreciably affecting the spread-spectrum content. The individual limiter outputs are then added back together, applied to the squaring circuit, and processed as previously described.
A more elegant approach to minimizing the effects of narrow-band signals is also described in our earlier patent. In this approach, the input signal is applied to a compressive receiver, which compresses narrow-band signals in time so that the compressive-receiver outputs resulting from different-frequency narrow-band signals are separated in time. Accordingly, a single limiter can be used to minimize the effects of narrow-band signals of all frequencies within the compressive-receiver frequency band. The limiter output is applied to a dispersive delay line whose relationship of delay to frequency is the reverse of that in the compressive receiver, so the compressed components are re-expanded before they are applied to the squaring circuit. The processing then proceeds in the manner described above.
The present invention is directed to the use of these principles on the outputs of a multi-element antenna array. Such arrays are used to achieve high gain or to perform direction finding. In principle, application of the teachings of our earlier patent to multi-element arrays can be fairly straightforward; it is only necessary to process each of the plurality of antenna-element outputs in the manner described in our earlier patent for a single-element output.
To carry out such a conceptually straightforward system, however, significant effort must be directed to preserving the information contained in the phase relationships among the element outputs. Since the directionality and direction-finding accuracy of a multi-element antenna depend on the phase relationships between the signals from the various antenna elements, any difference between the phase shifts experienced by different element outputs can lead to significant system errors. This is particularly of concern when minimization of the contribution of strong narrow-band element-output components involves the use of compressive receivers; discrepancies in the delays associated with different dispersive delay lines are a potential source of phase error.
The practical problem presented is thus to ensure that the phase shift experienced by the output of a given element during the compression step tracks the phase shifts experienced by the outputs of the other elements of the array during the same step.